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Selected Human Recombinant Proteins

There are two main systems available for the expression of recombinant proteins; prokaryotic (bacterial) and eukaryotic (yeast or mammalian). Prokaryotic expression systems have several advantages including, cost, culture conditions, rapid cell growth, yield and relatively short expression time. However, if the protein is required for functional or enzymatic studies, prokaryotic systems may not be the most suitable, as many proteins will form insoluble aggregates known as inclusion bodies which after refolding may not retain their biological function. Furthermore, bacterial expression systems do not allow for any post-translational modifications to be made (e.g. phosphorylation) which may be necessary for biological activity.

Eukaryotic expression systems such as yeast, mammalian or baculovirus cells are often selected for eukaryotic genes, even when expressed under the control of prokaryotic vectors. The main reason is that bacterial cells are unlikely to recognize human or eukaryotic promoters and terminators furthermore prokaryotic cells frequently recognize the protein products of cloned eukaryotic genes as foreign and remove them. As mentioned earlier prokaryotes do not carry out the same kind of post-translational modifications as eukaryotes for example, a protein normally coupled to sugars in a eukaryotic cell will be expressed as a "naked" protein when cloned in a bacterial cell. The stability and or activity of the protein may be affected as a result of this. If using a prokaryotic expression system the gene of interest should not contain human/eukaryotic introns as these will not be recognized and may result in premature termination of the protein or incorrect folding/processing of the protein. Improper folding can lead to biologically inactive products.